diff --git a/tools/testing/selftests/net/mptcp/mptcp_join.sh b/tools/testing/selftests/net/mptcp/mptcp_join.sh
index 4604dd13a87e..f4812e820acf 100755
--- a/tools/testing/selftests/net/mptcp/mptcp_join.sh
+++ b/tools/testing/selftests/net/mptcp/mptcp_join.sh
@@ -15,6 +15,7 @@ timeout_test=$((timeout_poll * 2 + 1))
 capture=0
 checksum=0
 ip_mptcp=0
+check_invert=0
 do_all_tests=1
 init=0
 
@@ -59,6 +60,8 @@ init_partial()
 		fi
 	done
 
+	check_invert=0
+
 	#  ns1              ns2
 	# ns1eth1    ns2eth1
 	# ns1eth2    ns2eth2
@@ -216,15 +219,21 @@ check_transfer()
 	out=$2
 	what=$3
 
-	cmp "$in" "$out" > /dev/null 2>&1
-	if [ $? -ne 0 ] ;then
-		echo "[ FAIL ] $what does not match (in, out):"
-		print_file_err "$in"
-		print_file_err "$out"
-		ret=1
+	cmp -l "$in" "$out" | while read line; do
+		local arr=($line)
 
-		return 1
-	fi
+		let sum=0${arr[1]}+0${arr[2]}
+		if [ $check_invert -eq 0 ] || [ $sum -ne $((0xff)) ]; then
+			echo "[ FAIL ] $what does not match (in, out):"
+			print_file_err "$in"
+			print_file_err "$out"
+			ret=1
+
+			return 1
+		else
+			echo "$what has inverted byte at ${arr[0]}"
+		fi
+	done
 
 	return 0
 }